The invention relates to apparatus for measuring current flowing in an electrical conductor, whether DC or AC.
A conventional solution for measuring such a current, particularly when it is DC, consists in cutting the cable, in interposing a known resistance between the cut ends, and in measuring the potential difference across the terminals of the resistance.
That solution requires the cable to be cut and a resistive element to be inserted between its cut ends.
It is an object of the invention to provide apparatus of a type avoiding that requirement and for this purpose making use of the performance of recent voltage amplifiers and the possibility of digitally processing the analog data supplied by such an amplifier.
To this end, it is provided apparatus for measuring DC or AC current carried by a conductor, comprising a voltage differential amplifier having two inputs for connection to the conductor each at one of two spaced-apart points of the conductor thereof, constituting the ends of a segment and a measurement circuit. That circuit is apt to convert an output voltage of the amplifier into an electrical current value. It further includes a microcontroller and a programmable memory for storing a calibration table in digital form. It may further include a generator for generating a calibration current at a frequency sufficiently different from a frequency of the current to be measured for enabling the calibration current to be separated from the current to be measured by filter means, and means for determining or correcting the calibration table responsive to a voltage measured at the calibration current frequency.
With DC, the circuit provides the magnitude of the current directly. With AC, a converter is provided. The invention is particularly adapted to measuring high currents.
Such measurement apparatus avoids any need to cut the cable and does not have any incidence on the connections between successive lengths of the conductor.
Measurement is based on the relationship which exists between the output voltage Vs from the amplifier and the current I carried by the conductor:
Vs=(1+xcex5)A.Vc
with Vc=R.I
where:
I is the current in the conductor;
R is the resistance of the segment of conductor;
xcex5 is a coefficient that depends on static errors, is always much less than 1, and it stored; and
A is the current/voltage amplification factor.
When the temperature of the conductor is liable to vary sufficiently to have significant influence on the resistivity and thus on the resistance of the segment, provision is made for a temperature sensor and for a correction to be calculated that takes account of the way resistance varies as a function of temperature.
Static errors can be measured in a workshop when the measurement apparatus is definitively installed on the conductor. It is also possible to perform periodic calibration by providing in the apparatus a current generator delivering a calibration current of known magnitude at a frequency that enables the calibration current to be distinguished from the current that is to be measured, which current may be DC or AC. The voltage measured when passing a known current that comes from a generator which is not subject to temperature drift makes it possible to determine the required calibration factors and possibly correction factors as a function of the temperature. A temperature sensor is no longer necessary if calibration by means of the current generator is performed often enough or just before each measurement.
The circuits of the apparatus advantageously constitute an assembly which is molded onto the conductor segment after that segment has been stripped to allow the connections to be made thereto.